BACKGROUND & AIMS: The study aims were to estimate the prevalence of different types and frequencies of fecal incontinence (FI), describe demographic factors, and identify risk factors. METHODS: The National Health and Nutrition Examination Survey (NHANES) assesses health status in the civilian noninstitutionalized US population. The validated Fecal Incontinence Severity Index was added to NHANES in 2005-2006. Participants were 2229 women and 2079 men aged 20 years or older. FI was defined as accidental leakage of solid, liquid, or mucus at least once in the preceding month. Sampling weights were used to obtain prevalence estimates for the national population. Multivariate logistic regression identified independent risk factors. RESULTS: The estimated prevalence of FI in noninstitutionalized US adults is 8.3% (95% confidence interval, 7.1-9.5) and consists of liquid stool in 6.2%, solid stool in 1.6%, and mucus in 3.1%. It occurs at least weekly in 2.7%. Prevalence is similar in women (8.9%) and men (7.7%) and increases with age from 2.6% in 20 to 29 year olds up to 15.3% in participants aged 70 years and older. FI is not significantly associated with race/ethnicity, education, income, or marital status after adjusting for age. Independent risk factors in women are advancing age, loose or watery stools, more than 21 stools per week, multiple chronic illnesses, and urinary incontinence. Independent risk factors in men are age, loose or watery stools, poor self-rated health, and urinary incontinence. CONCLUSIONS: FI is a prevalent age-related disorder. Chronic diarrhea is a strong modifiable risk factor that may form the basis for prevention and treatment.